Travis: Made Travis file tolerant against default job issue

Details:

* We define the job matrix explicitly with include statements, so
  that we have granular control over which combinations will run.
  Also, an explicit job matrix is required for OS-X support on the
  public Travis.

  When defining the job matrix explicitly, there are Travis environments
  that produce an additional default job. See these Travis issues:
    travis-ci/travis-ci#1228
    travis-ci/travis-ci#4681
    travis-ci/travis-ci#9843
  The public Travis does not seem to have this issue anymore,
  but Travis@IBM does have this issue (as of 9/2018). The workaround for
  this issue is to define variables globally and to exclude this same
  variable value in the matrix definition. Experiments have shown that
  not all variable combinations work. Using a combination of 'language'
  and 'os' set to the default values works.

* TODO: OS-X is not supported on Travis@IBM, so the inclusion of
  OS-X into the job matrix has been made conditional.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

.travis.yml

@@ -13,20 +13,43 @@ notifications:
 # and an empty 'python' variable in order to prevent that Travis attempts
 # to install Python.
 # TODO: Figure out how specific versions of Python 3.x can be used with OS-X
+
+# When defining the job matrix explicitly, there are Travis environments
+# that produce an additional default job. See these Travis issues:
+#   https://github.com/travis-ci/travis-ci/issues/1228
+#   https://github.com/travis-ci/travis-ci/issues/4681
+#   https://github.com/travis-ci/travis-ci/issues/9843
+# The public Travis does not seem to have this issue anymore,
+# but Travis@IBM does have this issue (as of 9/2018). The workaround for
+# this issue is to define variables globally and to exclude this same
+# variable value in the matrix definition. Experiments have shown that
+# not all variable combinations work. Using a combination of 'language'
+# and 'os' set to the default values works.
+
+# See note about explicit job matrix, above.
+language: ruby
+os: linux
+
 matrix:
+
+  # See note about explicit job matrix, above.
+  exclude:
+    - language: ruby
+    - os: linux
+
   include:
-#    - os: linux
-#      language: python
-#      python: "2.7"
-#      env:
-#        - PACKAGE_LEVEL=minimum
-#      cache: pip
     - os: linux
       language: python
       python: "2.7"
       env:
         - PACKAGE_LEVEL=latest
       cache: pip
+#    - os: linux
+#      language: python
+#      python: "2.7"
+#      env:
+#        - PACKAGE_LEVEL=minimum
+#      cache: pip
     - os: linux
       language: python
       python: "3.4"
@@ -75,13 +98,13 @@ matrix:
 #      env:
 #        - PACKAGE_LEVEL=latest
 #      cache: pip
-    - os: osx
-      language: generic
-      python:
-      env:
-        - PACKAGE_LEVEL=minimum
-        - PYTHON=2
-      cache: pip
+#    - os: osx
+#      language: generic
+#      python:
+#      env:
+#        - PACKAGE_LEVEL=minimum
+#        - PYTHON=2
+#      cache: pip
 #    - os: osx
 #      language: generic
 #      python:
@@ -96,15 +119,16 @@ matrix:
 #        - PACKAGE_LEVEL=minimum
 #        - PYTHON=3
 #      cache: pip
-    - os: osx
-      language: generic
-      python:
-      env:
-        - PACKAGE_LEVEL=latest
-        - PYTHON=3
-      cache: pip
+#    - os: osx
+#      language: generic
+#      python:
+#      env:
+#        - PACKAGE_LEVEL=latest
+#        - PYTHON=3
+#      cache: pip
 
 before_install:
+  - env | sort
   - if [[ "$TRAVIS_BRANCH" == "manual-ci-run" ]]; then
       export _NEED_REBASE=true;
     fi
@@ -115,6 +139,14 @@ before_install:
 
 # commands to install dependencies
 install:
+
+  # The following statement is a safety net in case the matrix exclusion
+  # does not work for some reason.
+  - if [[ "$TRAVIS_LANGUAGE" == "ruby" ]]; then
+      echo "Exiting from unwanted default Ruby job";
+      exit;
+    fi
+
   - if [[ "$TRAVIS_BRANCH" == "manual-ci-run" || "$TRAVIS_PULL_REQUEST_BRANCH" == "manual-ci-run" ]]; then
       export _MANUAL_CI_RUN=true;
     fi